In previously disclosed fluid heat exchangers of this type, the barrel was commonly capped at its ends, and then the end caps were interconnected by hoses or the like with intake and discharge manifolds through which the barrel could be serviced with the fluid undergoing heat exchange. When under pressure, the barrel was vulnerable to losing one or the other or both of its caps, and because of the change in diameter from the barrel to the manifolds, the fluid undergoing heat exchange was often unable to fully drain from the barrel when the pressure on the barrel was relieved.